Water vapor interference is a common problem in the analysis of air samples for chemical composition. Water vapor interferes with the analysis of the air samples for trace chemical species by masking the signal(s) for the element(s) or compounds(s) being tested for. Although water vapor interference effects a variety of analytical techniques including mass spectrometry and gas chromatography, it has the greatest impact on mid-infrared spectroscopy of trace analytes.
Water vapor is a particularly bad problem for mid-infrared spectroscopy because water vapor produces strong interfering absorption bands in several regions of the 4,000–400 cm−1 (2.5–25 μm) mid-infrared spectral range. In fact, the two strong water absorption bands located at 4,000–3,000 cm−1 and 2,300–1,300 cm−1 cloud over 50% of the mid-infrared spectral range making it very difficult to measure trace chemical species in this spectral range.
Water vapor interference becomes more pronounced in humid conditions. At conditions of 80% or higher relative humidity (and 25° C.), the concentration of water vapor is 2.5% or 25,000 ppm by volume (partial pressure). The interference of high concentrations of water vapor is greatest in situations where trace concentrations of chemical species are being measured. This is especially true when one is trying to measure chemical species in the concentration range of 200 ppm–1 ppb.
The problem of water vapor interference is compounded by the use of infrared absorption pathlengths in the range of 1–50 m which are typical when measuring analytes with concentrations of 200 ppm–1 ppb partial pressure. Gas phase analyte commonly measured by mid-infrared spectroscopy with 1–50 m absorption pathlengths for which water vapor produces substantial interference include volatile organic compounds and fossil fuel combustion products such as NO2, NO, SO2 and NH3. At such long pathlengths, water interference can reduce infrared light transmission to less than 10%. Low level of infrared transmission results in large reductions in the signal/noise level of the infrared absorption measurements of gas phase analytes.
One approach to reducing interference from water vapor is to subtract a re-scaled version of a reference water spectrum for only water plus air from the sample absorption spectrum. The theory behind this approach is that removing the reference water/air spectrum will reveal an un-obscured sample spectrum, however, this approach has several significant limitations.
First, the detailed vibrational-rotational structure of water absorption bands are very sensitive to both the partial pressure of the water vapor and the partial pressure of other species present at higher concentrations such as CO2 (typically present at concentrations of around 400 ppm). Therefore, for the subtraction method to work effectively the partial pressure of the water vapor and the other major components in the reference spectrum must match that of the sample spectrum. Differences in the water partial pressure between the sample and reference spectrum results in differences in the spectral widths, height and shape of the water sub-band peak due to the vibration-rotational structure which arise from collisional line broadening effects. When differences in spatial widths, height, and shape of infrared spectral absorption features occur, the subtraction of the reference spectrum can only partially remove the water band interference.
Second, because water vapor is still physically present in the sample, water vapor interference can still reduce infrared light transmission to less than 10% (for 1–50 m absorption pathlengths) in many mid-infrared regions. This loss in light transmission limits the detection sensitivity for trace atmospheric constituents even when the water absorption features can be successfully removed by digital subtraction of a water vapor reference spectrum.
A second approach to reducing water interference is to pass the sample gas through a chemical filter which removes water through a chemical reaction such as a hydration reaction. A common water filter agent using a hydration reaction is anhydrous MgSO4. The primary problem with this approach is that the chemical drying agent will also remove a substantial portion (greater than 30%) of the trace analytes of interest in the air sample.
A third approach for reducing water vapor includes passing the sample gas through the inside of a 1 m or greater length porous Nafion membrane (type of fluro-sulfnonate polymer) tubing while a countercurrent flow of dried nitrogen or dried air is maintained over the outside of the tubing. This approach is not always practical because of the requirement of a pressurized source of water-free nitrogen or air. This approach is particularly impractical for portable measurement systems.
A more successful approach to combating water interference is to use a thermoelectrically cooled cold trap which is cooled to a sufficiently low temperature to remove a large fraction of the water vapor present without removing any significant amount of the trace analytes (analyte(s)=the compound(s) or element(s) in the sample being analyzed) present. Assuming thermal equilibrium has been reached, when water vapor is present at a concentration of about 20,000 ppm (2% by partial pressure) a reduction of 75 fold can theoretically be achieved by cooling the air to a temperature of −28° C. In application, the sample gas passes too rapidly through the cold trap for thermal equilibrium to be reached. Even so, experimental measurements have shown that a reduction in the water vapor of about 5 fold can be achieved with a portable thermoelectric cooling system that has a temperature of −28° C. and a gas flow rate of about 1.5 liter/min. This 5 fold reduction in the water vapor reduces the loss of infrared light by optical absorption from the water bands in a 1 m pathlength gas cell from a 87% loss down to a 28% loss. Substantial reductions in water vapor concentration can still be achieved by passing the sample gas through a thermoelectrically cooled water trap cooled to a temperature of −5° C.
In general, the concentration of the trace components in the air sample are not reduced by the cold trap if the equilibrium vapor pressure of the compound is above their partial pressures in the air sample. The partial pressure of a compound such as a volatile organic compound often needs to be lower than the equilibrium vapor pressure because of the condensation of azeothrophic mixtures between water and the analyte compound. This partial pressure condition is met for many compounds of interest including: benzene, toluene, xylenes, ethylbenzene, trichloroethylene, NO and NO2, when present in the sample at concentrations below 200 ppm.